1. Field of the Invention
This invention relates to the use of poly(alpha,beta-alkenylcarboxylates), such as di-isopropenyl esters, to crosslink polymers containing hydroxyl groups.
2. Description of Related Art
Polymers containing hydroxyl groups (e.g., acrylic polymers) are typically crosslinked with melamine-formaldehyde resins (see, for example, U.S. Pat. Nos. 3,843,390; 4,737,551; and 4,812,533. However, formaldehyde (a possible carcinogen) can be produced as a byproduct of such crosslinking reactions and so efforts have been made to reduce such use of melamines (see U.S. Pat. No. 5,102,954).
Other polymers containing hydroxyl groups (e.g., polyether polyols) are typically crosslinked using polyisocyanates (see, for example, U.S. Pat. No. 4,732,960. However, polyisocyanates are relatively toxic materials. (see U.S. Pat. No. 4,758,475).
Still other polymers containing hydroxyl groups (e.g., certain styrene-acrylic emulsion polymers) are typically crosslinked using coalescing aids (e.g., isobutyraldehyde trimers such as Texanol.RTM.) along with crosslinking agents. Such coalescing aids represent an additional volatile component and their elimination would reduce the number of volatile components utilized in producing such crosslinked products (see published European Patent Application 0 466 409 A1).
U.S. Pat. No. 4,020,123 discloses the use polyisopropenyl esters to crosslink polymers containing amino groups but does not disclose the use of polyisopropenyl esters to crosslink acrylic polymers containing hydroxyl groups. Polymers containing amino groups are generally not readily available and are more expensive than hydroxyl-containing polymers.
Stackman and Steinmann (Polym. Prepr., Amer. Chem. Soc., Div. Polym. Chem 9(1), 628 (1968); CA71:125036P discloses the preparation of thermoplastic polyamides from di-isopropenyl esters and diamines. The preparation of thermoplastic polyesters from di-isopropenyl esters and ethylene glycol is also disclosed in this article. In particular, this article discloses that the reaction between di-isopropenyl adipate and ethylene glycol gave a thermoplastic, low molecular weight polyester polymer and the reaction between di-isopropenyl sebacate and ethylene glycol also gave a thermoplastic, low molecular polyester polymer. Hence, this article does not suggest the use of di-isopropenyl esters as crosslinkers, particularly as crosslinkers to produces higher molecular weight thermoset polymers.
Therefore, it is an object of the present invention to provide an improved process for crosslinking hydroxyl containing polymers.
It is a further object of the present invention to provide a process for crosslinking hydroxyl-containing polymers which does not require the use of polyisocyanate crosslinkers or volatile coalescing aids or involve the evolution of formaldehye as a byproduct.
Other objects of this invention will be apparent from the following description thereof.